As is known in the art with regard to polymer electrolyte fuel cells having solid polymer electrolyte membranes, the wet state of the electrolyte membranes affects the output voltage of the fuel cells. There are known techniques of measuring the AC (alternating current) impedance of fuel cells to detect the various states of the fuel cells. For example, a technique disclosed in Japanese Patent Laid-Open Gazette No. 2003-86220 measures the AC impedance (complex impedance) of fuel cells to estimate the humid state inside the fuel cells, that is, the wet state of the electrolyte membranes. These conventional techniques measure the AC impedance of the fuel cells at a preset timing and estimate the wet state of the electrolyte membranes at the preset timing.
There is a new parameter that has yet not been noted for study or examination to determine the relation between the AC impedance of fuel cells and the wet state of electrolyte membranes. The inventors of the invention have noted this new parameter and found that the wet state of electrolyte membranes in fuel cells is detectable based on measurement of the new parameter.